It is known to simulate processing conditions and to perform measurements under the simulated processing conditions to determine how a particular material will react under actual processing conditions. For example, U.S. Pat. No. 4,426,160 of Couderc discloses a method and apparatus for optically measuring the deformation of a material (e.g., pitch) under heat to determine the wetting power of the material. Also, U.S. Pat. No. 3,292,418 of Oehme et al. discloses a method and apparatus that simulates the conditions (i.e., high temperatures) encountered in drying equipment employed with printing presses to obtain indications of blistering characteristics prior to printing. U.S. Pat. No. 4,989,991 of Pecot et al. discloses a method and apparatus for calibrating the emissivity characteristics of a semiconductor wafer by sensing the actual temperature of a semiconductor wafer with a thermal sensor and, at the same time, measuring the temperature of the wafer by optical pyrometry under processing conditions that ideally would be the same for all wafers to be processed in a processing chamber.
It is also known in the art to use optical techniques to determine the design or shape of an object. Kyle et al., U.S. Pat. No. 3,614,237, disclose a method and apparatus for measuring the contour of a surface by illuminating the surface through a periodically repetitive image structure, thereby casting a shadow of the structure onto the surface. The surface and the shadow cast thereon are viewed visually by an observer or are photographed through the structure as moire fringes. The contour of the surface is then determined from the moire fringes.
Crabb et al., U.S. Pat. No. 4,650,333, disclose a system for detecting printed circuit wiring defects and for measuring the height of circuit features. Non-uniformity and variations of the substrate surface due to bending or warpage can be accounted for when measuring the height of the circuit features. The substrate and circuit features are illuminated by an energy source and a scanner receives energy reflected from the substrate and the circuit features and generates a signal which varies in accordance with the intensity of the reflected energy. An analyzer receives the signal generated by the scanner and derives therefrom a measurement representative of the height of the circuit features relative to the substrate.
In attempting to determine the thermal warpage developed in printed wiring boards during assembly/manufacturing operations, some difficulties arise. Firstly, making direct measurements of the warpage in the printed wiring board during such operations is problematical inasmuch as the board typically is moved along a conveyor through one or more oven zones to facilitate soldering, with the oven enclosures restricting access to the board. Thus, making a direct measurement of the warpage in printed wiring boards in process may be impractical. Secondly, in attempting to simulate actual process conditions using a test station, it can be difficult to change the temperature in the test station quickly enough and accurately enough to adequately simulate the actual process conditions existing along the conveyor. Thirdly, even if one can adequately replicate the process conditions of the conveyor within a stationary work station, the task of accurately measuring the warpage in a given board remains a daunting one.
Accordingly, it can be seen that a need yet remains for a method and apparatus for measuring thermal warpage in test devices, elements, or specimens (especially in printed wiring boards) which accurately simulates actual process temperature conditions and which attains accuracy in measuring the thermal warpage. It is to the provision of such a method and apparatus that the present invention is primarily directed.